1. Technical Field
Aspects of this document relate generally to the ignition systems. More particular implementations relate to high-power breakdown spark plugs for internal combustion engines.
2. Background
To ensure the ignition of lean/diluted mixtures in modern advanced internal combustion engines, high-energy spark ignition systems and novel spark plugs are used to initiate and promote the ignition process. A spark discharge process can be divided into a breakdown period and a continuous discharge period. The breakdown period is identified as capacitive discharge, characterized by short duration and high peak current. On the other hand, the subsequent continuous discharge period is a resistive discharge with relatively long duration and low current.
A step-up transformer of the ignition coil boosts the voltage to break down the spark gap; however, the energy discharged during breakdown comes from the near-gap capacitors, which are charged during a pre-breakdown voltage build-up process. The capacitive discharge during breakdown is highly dynamic, lasting only on a time-scale that is measured in the nanoseconds range. The characteristics of the step-up transformer, such as the turn ratio and the impedance of the windings, affect the breakdown process insignificantly but determine the characteristics of the continuous discharge period.
Although the breakdown process delivers only a very low portion of the overall ignition energy, due to its short duration, the features of high voltage and high current are favorable to ignite lean and diluted mixtures. Thus, enhancement of the capacitive discharge energy during the breakdown process can significantly promote the ignition process. To increase the capacitive discharge energy an increase of the near-gap capacitance is useful. A spark plug forms a virtual concentric cylindrical capacitor, with the outer surface of the center electrode and the inner surface of the metal shell as the conductors, and the insulation ceramic as the dielectric media. The spark plug insulator is normally made from alumina (Al2O3), which possesses excellent mechanical strength and thermal conductivity, and which has been employed commonly for internal combustion engines. Due to the relatively low dielectric constant of alumina (˜10) and the poor metal-ceramic surface contact of a conventional spark plug structure, the capacitance of a conventional spark plug ranges from about 10-20 pF, providing up to about 2-3 mJ of breakdown energy.